Heat 2 tbsp oil in a large pan. Slice the onion and add to the pan. De-seed and chop the chillies, chop ginger and add to the pan. Crush the garlic and add to the pan; cook for 2-3 minutes.Add chili powder, turmeric, garam masala and sugar; cook for 1-2 minutes.Add tomato puree and tinned tomatoes; stir and cook for 7-11 minutes.Transfer sauce to a food processor and blitz until smooth.Add 1 tbsp fresh oil into the pan in which you cooked the sauce and fry the chicken until lightly coloured. Pour the sauce back into the pan and add the curry leaves. Simmer for 10 minutes, or until chicken is cooked.Stir in the yogurt with half of the chopped coriander and serve, garnishing with the remaining coriander.